Abstract: An optoacoustic technique for absolute, accurate, continuous, and real-time measurement of a variety of important diagnostic variables is disclosed, where the variables include: (1) noninvasive measurements of circulating blood volume (BV) and cardiac output (CO); (2) calculation from the measured variables of cardiac index (CI) and systemic oxygen delivery (DO2); (3) concentrations of hemoglobin derivatives (e.g., carboxyhemoglobin [HbCO], reduced hemoglobin [Hb], oxygenated hemoglobin [HbOxy], and methemoglobin [HbMet]), total hemoglobin concentration [THb], concentrations of lactate, myoglobin, cholesterol, body pigments, and exogenous dyes; (4) content in tissues of water, fat, protein, calcium, and blood; as well as density of hard and soft tissues; and (5) accurate noninvasive measurement of blood pressure (or vascular pressure) in arteries, arterioles, veins, capillaries, using occlusion-induced changes in optoacoustic signal induced in blood circulating in the vessels.